Investment in Unconsolidated Entities (Details) (USD $)	9 Months Ended				12 Months Ended		0 Months Ended		12 Months Ended	1 Months Ended	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Construction and other related party loans	Dec. 31, 2012 Construction and other related party loans	Dec. 31, 2012 The Shops at Mission Viejo	Dec. 31, 2012 IMI Woodfield Mall	May 21, 2012 Klepierre	Mar. 14, 2012 Klepierre	Dec. 31, 2012 IMI Joint Venture Woodfield Mall	Jan. 31, 2013 IMI Joint Venture The Shops at Mission Viejo	Dec. 31, 2012 IMI Joint Venture The Shops at Mission Viejo	Dec. 31, 2012 IMI Joint Venture IMI Woodfield Mall	Dec. 31, 2012 IMI Joint Venture IMI The Shops at Mission Viejo	Sep. 30, 2013 United States property	Dec. 31, 2012 United States property	Sep. 30, 2013 Japan property	Dec. 31, 2012 Japan property	Sep. 30, 2013 South Korea property	Dec. 31, 2012 South Korea property	Sep. 30, 2013 Mexico property	Dec. 31, 2012 Mexico property	Sep. 30, 2013 Malaysia property	Dec. 31, 2012 Malaysia property	Jan. 09, 2012 Italy GCI property
Schedule of Equity Method Investments
Number of properties														74	78	9	8	3	2	1	1	1	1
Number of properties sold																								45
Joint venture ownership percentage								28.70%
Joint venture ownership percentage after transactions							28.90%		50.00%		51.00%	50.00%	49.00%
Loans to related party			$ 103,600,000	$ 25,400,000
Joint venture ownership percentage before transaction					100.00%	100.00%
Gain due to formation of joint venture					0
Aggregate face amount of mortgage notes									425,000,000	295,000,000
Interest rate on mortgage loan (as a percent)									4.50%	3.61%
Distribution from joint venture financing proceeds	$ 143,202,000	$ 114,508,000								$ 149,700,000